The solid state chemistry of bis(dimethylamino) polymethine salts (n = 1,2,3,4) was investigated using thermogravimetry, difference thermal analysis, mass spectroscopy, UV/VIS spectroscopy and HPLC to analyze the extruded gaseous products and the residue. The observed pathway is nonamethine → heptamethine → pentamethine → trimethine → insoluble residue extruding dimethylamine and ethylene. The temperature of the strong exothermal process and the thermal behaviour in detail depend on the chain length, the anion and the crystal structure.